Insulating the roof rafters

You can install a shallow 6" can made for 2x6 construction. Halo H27ICAT is the 6" you can use. Lots of manufacturers make these shallow cans.

You can use regular incandescent, fluorescent and LED lamps and have a large selection of trims. The only drawback is you can't use taller lamps or they might stick out the bottom. The most common is the 50PAR30 / 75R30 style lamps. These are also made in remodel style in case the ceiling is already up.

Here is the spec sheet for the H27

EDIT

I'll give this another try. I originally deleted this answer because it is not a slope can. Where I work, some people use regular can lights and use adjustable trims. Because of the depth you have, 8 inches, this can is 5-1/2 inches. I suggest you use a gimbal ring trim. It works like a eyeball, but the lamp is recessed so you don't get that 'bug eye' some people associate with eyeball trims. You can get the above can in 5 inch or 6 inch round cans, both IC rated, depending on the manufacturer your supplier uses. Think about a trim like this.

Make sure you use the same manufacturer as the can and trim to cover yourself. This gives you 2-1/2 inches for your channel and insulation. If this doesn't float your boat try going to a local lighting showroom and look at what they might have. If you want recessed you will find what you want. Consider LEDs to keep from getting the ladder out if your slope gives you a tall ceiling. While considerably more, doing it all together will be less painful. Either way PAR30 Halogen or PAR30 LEDs offer different beam spreads, so you can at least do something with the lighting.The biggest problem is you can't shine the light back to the wall, mostly straight down or with the angle the can is hung, not backwards. If you choose this, don't forget the dimmers!

Definitely close off the ceiling rather than plugging the vents, IMHO.

The "fanfold" insulation [typically 1/4" X 4ft X 50 ft - R1 possibly some thicker versions available, on a quick look around - thicker would of course be higher R-value, as well] intended for use under siding (and sometimes roofing) might be another option, in larger sheets (less joints to seal.) More than just vapor barrier...and it might support more serious insulation (just not too much) above it.

I presume that you recognize and accept that until you do drywall over it, this (or vapor barrier, or XPS sheets, etc...even exposed fiberglass, to an extent) is a fire hazard. In a similar situation, I'm putting my effort towards getting the drywall on the ceiling so I can insulate the heck out of the ceiling - a 5000W heater does not go very far, otherwise. Specifically, 5KW * 3413 btu/kWh = 17065 btu/hr (input) - .vs. 720 sqft (30x24) of R1 (1 Hr x sq-ft x degree F/BTU) ceiling would only net you 23.7 degrees (F) (17065/720) above outside ambient if that were the ONLY place heat was lost (floor, walls, doors all infinitely insulating, and zero air exchange - obviously an incorrect assumption - just illustrating the point.)

If the walls are 8 ft high, amounting to 864 sq ft, plus the 720 sq ft of ceiling, and we assume that all are insulated to R11 (optimistic for many garage doors and all regular doors & windows - but low for most modern walls and ceilings) the same 5KW could heat to 118 F above outside ambient (but I'm still ignoring the floor and air exchange...)